As is known, splitting a thin film may be achieved by implantation of chemical species in a source substrate, for example of silica, to induce the formation of a zone of defects at a particular depth. These defects may be micro-bubbles and/or platelets and/or micro-cavities and/or dislocation loops and/or other crystalline defects, locally disrupting the crystalline quality of the material; their nature, density and size are strongly dependent on the species implanted (typically hydrogen) as well as on the nature of the source substrate. A heat-treatment may then be applied to enable the development of specific defects present in the weakened zone, which enables splitting of the thin film from the source substrate to be achieved later, in principle by subjecting the defects to pressure. This has in particular been described in the U.S. Pat. No. 5,374,564 and developments thereof, such as described in U.S. Pat. No. 6,020,252.
Splitting may be made, generally further to heat treatment, by applying an external force which induces fracture in the weakened zone until splitting is achieved of the thin film. When the splitting is produced at high temperature (typically near approximately 500° C.) among the technological problems encountered, mention should be made of the roughness of the surface as well as the degradation of the film transferred during the thermal splitting. This renders the treatment steps that follow more difficult. For example, the transferred film must be polished more and there is a risk of crystalline defects being created during the following treatments. Furthermore, in heterostructures (comprising a superposition of substrates of different materials), another technological problem encountered is the presence of a field of very high stresses in the various films in contact, during the heat treatment, due to the difference in the coefficients of thermal expansivity of the various materials placed in contact. This may induce the degradation of the heterostructures if the thermal splitting occurs at a temperature higher than a critical temperature. This degradation may, typically, be the breakage of one or both substrates brought into contact and/or are the unbending of the substrates at the bonding interface. This is the reason why it may be desired to achieve the splitting at lower temperature.
One way to achieve splitting at low temperature is to adjust the implantation conditions. For example, an excess dose of the implanted species may increase the weakening of the implanted zone in which the splitting at low temperature occurs. For example, Henttinen et al. showed that, if the source substrate is a wafer of silicon, a dose of hydrogen ions implanted at 1×1017 ions/cm2 (i.e., a molecular hydrogen does of 5×1016 ions/cm2), enables splitting by a mechanical force after performing the following steps: treatment, as for the target substrate, by a plasma chemical activation; cleaning of RCA1 type, bonding at ambient temperature of the source substrate onto the target substrate, and annealing at 200° C. for two hours. (K. Henttinen et al., Applied Physics Letters, Volume 16, Number 17, 24 Apr. 2000; pp. 2370-2372). The mechanical force utilized came from a blade inserted at the bonded interface to initiate the splitting.
This approach, although reducing the roughness of the transferred surface by of the order of half with respect to conventional splitting solutions that are purely thermal and without plasma activation, involves a step of plasma chemical activation followed by wet chemical cleaning known to those skilled in the art as RCA1 cleaning, which may represent a significant drawback from an industrial point of view, such as increased cost. Moreover, it is important to note that, due to the high implant dose of hydrogen, the heat treatment after bonding must not exceed 300° C., the temperature at which thermal splitting could occur, in which case the aforementioned advantage of the reduction in roughness of the transferred surface would not be achieved. A step of plasma chemical activation, followed by RCA1 cleaning is thus indispensable for reinforcing the bonding before its thermal consolidation according to the technique described by Henttinen et al.
It is important to note that, in fact, it is not solely the temperature of treatment which affects the later conditions of splitting of the thin film, but also the duration of that treatment, which has resulted in the concept of thermal budget, for example, as described in French patent application no. FR-2 767 416. Mechanical energy can also be provided, for example by a tool of “guillotine” type as described in PCT patent publication no. WO 02/083387.
Thus, it has been found that, if the thermal budget is too low, the transfer of the thin film is of poor quality, whereas if it is too high, fracture may occur of one of the substrates in the case of a heterostructure. It can thus be understood that in principle there exists a narrow window for the operational parameters, such as the implanted doses, the nature of the materials, the temperatures of annealing, and the like. The narrow operational parameters, however, constitute a heavy constraint for industrial exploitation.
Furthermore, the mechanical splitting often consists of introducing one or more blades from the edges of the structure, as if to “cut it out” along the weakened zone; the term ‘assisted splitting’ is sometimes used, since the role of the tool (such as a blade) is to propagate the fracture wave from one edge of the structure to the other.
This type of fracture leads to the following defects, at the future surface freed by the splitting of the thin film: 1) crown defect (non-transferred zone, at the periphery of the final product), for example related to a local bonding energy too low with respect to the rest of the interface, and to the introduction of tools to start off the transfer; 2) lack of uniformity (low frequency roughness) of the thickness of the thin film transferred, in particular due to the fracture wave assisted mechanically, thus irregular, by fits and starts, which then necessitates treatments, such as polishing, which it is however generally sought to avoid, 3) difficult industrial implementation, given the use of a tool which accompanies the propagation of the fracture, which implies an individual treatment of each structure (or wafer).
Most of these drawbacks are found in the case of the splitting of a thin film in a homogeneous substrate (with a single component material (SOI, for example). The splitting of the thin layer is of course also determined by the choice of the chemical species implanted.
It was indicated above that hydrogen is generally implanted, but other options have been proposed, in particular by implanting helium. A combination may even be made of two different chemical species. For example, Agarwal et al. found that implanting both hydrogen and helium enabled the total implanted dose to be reduced. (A. Argawal et al. Applied Physics Letters, volume 72, Number 9, 2 Mar. 1998; pp. 1086-1088). The dose effect is apparently due to the different roles played by hydrogen and helium: the hydrogen interacts with the Si—Si bonds broken by the implantation, to create Si—H bonds, resulting in a high density of platelet type defects of a size of the order of 3-10 nm (termed H-defects of platelet type), whereas helium, which does not act chemically, leads to the apparition of a lower density of larger defects (size greater than 300 nm approximately). The heat treatments envisaged by Agarwal et al. are 450° C. for 20 minutes or 750° C. for 20 seconds, which necessarily implies the drawbacks mentioned above in relation to splitting at high temperature.
This hydrogen-helium combination has also been studied, in a more theoretical manner, by Cerofolini et al., who noted that pressurization of the defects was greater with the implantation of helium than with that of hydrogen, and that the heat treatment could have different effects according to the temperature chosen. (G. F. Cerofolini et al., Materials Science and Engineering B71, 2000, pp. 196-202). Cerofolini et al. reported that annealing between 150° C.-250° C. leads to a reduction in the number of Si—H bonds, and annealing in the range 300° C.-450° C. leads on the contrary to an increase in that number, whereas annealing above 550° C. tends instead to reduce that number again. However, Cerofolini et al. do not deduce practical conclusions therefrom as to the manner of obtaining thin films of good quality (in particular in relation to the state of the surface) for a moderate cost.